This invention relates to a watercraft propulsion system and more particularly to an improved jet propulsion unit and driving arrangement therefor for such watercraft.
The use of so-called "jet propulsion units" is highly advantageous in watercraft, particularly a type of watercraft known as a "personal watercraft," although the use is not so limited. The advantage of the jet propulsion unit is that it can be concealed within the hull of the watercraft, and also it permits operation in very shallow water.
These jet propulsion units operate by drawing water from the body of water in which the watercraft is operating by an impeller that is driven by a prime mover generally mounted in the hull of the watercraft. The water is then discharged rearwardly through a discharge nozzle so as to provide a propulsive force for the watercraft. By mounting a pivotally supported steering nozzle in communication with the jet propulsion unit discharge nozzle, it is also possible to redirect the spray to the right or left and thus effect steering of the watercraft.
In connection with these jet propulsion units, generally the impeller shaft and the discharge nozzle are coaxial. It is important, however, to mount the discharge nozzle so that it has a relatively shallow angle to the horizontal, if any, to avoid providing either a lifting force or a porpoising force on the watercraft during its travel through the body of water. Because of the aforenoted coaxial relation of the impeller shaft and discharge nozzle axes, the angle of the discharge nozzle generally dictates the angle of the impeller shaft. Since the impeller shaft is generally driven by a direct drive from an engine output shaft, this also affects the actual mounting position of the engine in the watercraft hull. This is not particularly desirable for some reasons which will be discussed later.
It is, therefore, a principle object of this invention to provide a water jet propulsion unit for a watercraft wherein the discharge nozzle is maintained at the appropriate angle for the desired hull effect, but the impeller shaft for driving the impeller is positioned at a different angle to optimize the engine location.
One reason why the engine location in the hull is important is that it is desirable to maintain a relatively low center of gravity for the engine. However, the normal impeller shaft angle can result in too low an engine mounting. This presents a problem in connection with four cycle engines of the type that employ wet sump crankcases. The low mounting of the engine may not afford adequate oil capacity for use with a wet sump engine. Of course, it is possible to use dry sump engines, but they become more complex and costly.
It is, therefore, a still further object of this invention to provide an improved jet propulsion system for a watercraft wherein the engine can be mounted in the watercraft at a height that affords adequate oil capacity for a wet sump engine and yet wherein the discharge nozzle of the jet propulsion unit is disposed at the appropriate angle relative to the hull to provide the desired trim or hull effect.